Nitric oxide radicals and their reactions

Ernst van Faassen1 and Anatoly F. Vanin2

1Debye Institute, Section Interface Physics, Ornstein Laboratory, Utrecht University, 3508 TA,

Utrecht, The Netherlands 2Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991,

Russian Federation

Nitric oxide is a peculiar radical species in many respects. It is a radical in the sense that its electronic configuration contains a half-occupied orbital that is occupied by a single electron only. In spite of this open electronic structure, its reactivity with most other molecules is surprisingly low (notable exceptions being superoxide radicals). The half-occupied orbital provides the molecule with a nonzero total electronic angular momentum S = 1/2 and nonzero electronic magnetic moment from the electrons. However, the ground state of NO^ is not paramagnetic at all. This diatomic molecule has fifteen electrons in an electronic configuration (K2K2) — (2sa)2(2sa*)2(2pn)4(2pa)2(2pTC*)1 and rotational symmetry around the molecular axis. The unpaired electron is located in one of the antibonding orbitals. The axial symmetry of the electronic fields allows for the projections of total spin S and angular momentum L along the molecular symmetry axis as conserved quantities. As such, NO^ is a good example of a type (a) molecule in Hund's classification, where the axial projections of S, L and J = L + S are given the quantum numbers S, A and £ = A + S respectively. Type (a) molecules typically have strong internuclear fields to constrain L and strong spin-orbit coupling to constrain S to the molecular axis. Free NO has S = 1/2 and A = 1. The ground state 2S+1 A^ = 2n/ has antiparallel coupling, with the parallel coupled 2n3/2 state being 124.2 cm-1 ~ 15 meV higher in energy. Rotationally excited ladders of these states appear with energy spacings of about 5 cm-1. The magnetic moment ^ = is proportional to the gyromagnetic ratio g£. For molecules of Hund's type (a), the gyromagnetic ratio is given by [1]

We note that the ground state 2n1/2 of this radical has nonzero angular momentum (A = 1) but zero total magnetic moment. In other words, it is not paramagnetic at all when in free state. As such, the true ground state of an isolated NO^ molecule in the gas phase cannot be detected by magnetic resonance spectroscopy.

However, detection of NO radicals with electron paramagnetic resonance (EPR) becomes possible under certain conditions. In dilute NO^ gas at room temperature (kT ~ 25 meV), ca 70% of the molecules are thermally excited to the 2n3/2 state with = 4/5. This state shows linear Zeeman splitting when brought into an external magnetic field. At X-band frequencies ~10 GHz, the EPR transitions occur at magnetic fields near 9100 G. The EPR spectrum (Fig. 1) of this dilute gas is ca 250 G wide and shows 9 well resolved lines. It appears as a triplet of triplets with 3:4:3 intensity. The larger splitting is caused by the zero-field splitting (ZFS) interaction and the smaller splitting by the hyperfine coupling to the magnetic moment of the 14N nucleus (I = 1). At higher gas pressures, the lines are broadened by spin-spin interactions, and the resolution of the ZFS is lost.

Alternatively, NO^ molecules may be adsorbed on solid surfaces, where the interactions with atoms of the substrate lead to quenching of the orbital angular momentum. The resulting magnetic moment is then determined by the electronic spin only [3]. Finally, NO^ often appears as a ligand in paramagnetic metal complexes with Co or Fe centers. Well-known examples are the ferrous nitrosyl complexes with heme moieties or with iron-dithiocarbamate complexes as discussed in Chapters 2-5 and 18. The small difference in electronegativity between nitrogen and oxygen gives NO^ a modest electrical dipole moment of 0.159 D, i.e. more than an order of magnitude smaller than that of water.

Nitric oxide gas is colorless in the visible wavelength region, but has a prominent infrared absorption at 1878 cm-1 = 0.233 eV due to the fundamental vibrational band [v(N—O) stretch mode] [4,5]. This characteristic absorption corresponds to a wavelength of 5.3 ^m and is often used for detection of NO in the gas phase with optical sensors [6]. For example, the infrared absorption line has been used to detect traces of NO escaping from biological samples at rates above ca 10 pmol/s [7]. Upon binding to, for example, the metal center of a complex, the frequency of this vibrational band is changed considerably by a combination of two antagonistic effects. First, the anchoring to the heavy metal center raises the effective reduced mass of the stretch vibration and lowers the frequency. Additionally, the strength of the NO bond is affected by partial charge transfer of the unpaired electron towards the metal.

Fig. 1. X-band EPR spectrum of 1 Torr NO gas at room temperature. The scan range is 335 Gauss. (From Ref. [2].)

This transfer of the electron away from the antibonding orbital significantly stiffens the NO bond and tends to raise the vibration frequency. This stiffening is the reason that the stretch vibrations of free nitrosonium NO+ are raised to 2200 cm-1 [8]. Conversely, transfer of additional electron density into the antibonding orbital lowers the frequency of the v(N—O) stretch mode to ca 1363 cm-1 [9] for the free nitroxyl anion NO-. IR and Raman spectroscopy of nitrosyl complexes have shown in accordance that the nitrosyl stretch vibrations span the full range of 1100-2000 cm-1, and the stretch vibration of the NO ligand was found to be a good spectroscopic indicator of the degree of charge transfer in nitrosyl complexes with transition metal ions [10]. Prime examples are the endogenous nitrosyl-iron complexes in biological systems like tissues or blood. The extent of charge transfer is directly related to an important structural parameter, namely the orientation of the nitrosyl axis with respect to the metal ion. The MM—N—O bond angle is predicted to be 120° for NO- ligands, whereas a nearly linear alignment is found for nitrosyl cations. Such charge transfer was found to be very characteristic for nitrosyl ligands on iron atoms and the shared nature of the unpaired electron is accounted for in Enemark-Feltham notation [11] for the combined electronic configuration of the Fe—NO motif. Paramagnetic mononitrosyl iron complexes (MNICs, cf Chapter 18) have {FeNO}7 configuration and typical frequencies of the v(N=O) stretch mode are 1670-1720 cm-1. Paramagnetic dinitrosyl iron complexes (DNICs, cf Chapter 2) have {Fe(NO)2}7 or {Fe(NO)2}9 configuration and typical v(N=O) stretch frequencies are higher 1730-1800 cm-1. Significantly, the two nitrosyl ligands are found to stretch with slightly different frequency, separated by 30-60 cm-1. For example, for Cys-DNIC the two stretch frequencies are reported as 1730 and 1770 cm-1 [12]. Therefore, the two ligands show distinct charge transfer and net effective charge. This nonequivalence of the nitrosyl ligands is highly significant as it seems the reason for the unusual reaction chemistry of DNIC as described in Chapter 2. A selection of experimental stretching frequencies for nitrosyl-metal complexes can be found in Refs. [10,11,13,14].

For covalent nitroso compounds the v(N— O) stretch mode [18] is a good marker as well (cf Table 1).

Table 1 Typical frequencies for the v(N—O) stretch mode in various compounds

Remarks

Refs.

N^Fe3+-DETC

N^Fe2+-DETC NO—Fe3+-porphyrins NO—Fe2+-porphyrins S-nitroso

N-nitroso aliphatic N-nitroso aromatic O-nitroso

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